Site-selective protonation enables efficient carbon monoxide electroreduction to acetate

Electrosynthesis of acetate from CO offers the prospect of a low-carbon-intensity route to this valuable chemical––but only once sufficient selectivity, reaction rate and stability are realized. It is a high priority to achieve the protonation of the relevant intermediates in a controlled fashion, and to achieve this while suppressing the competing hydrogen evolution reaction (HER) and while steering multicarbon (C2+) products to a single valuable product––an example of which is acetate. Here we report interface engineering to achieve solid/liquid/gas triple-phase interface regulation, and we find that it leads to site-selective protonation of intermediates and the preferential stabilization of the ketene intermediates: this, we find, leads to improved selectivity and energy efficiency toward acetate. Once we further tune the catalyst composition and also optimize for interfacial water management, we achieve a cadmium-copper catalyst that shows an acetate Faradaic efficiency (FE) of 75% with ultralow HER (<0.2% H2 FE) at 150 mA cm−2. We develop a high-pressure membrane electrode assembly system to increase CO coverage by controlling gas reactant distribution and achieve 86% acetate FE simultaneous with an acetate full-cell energy efficiency (EE) of 32%, the highest energy efficiency reported in direct acetate electrosynthesis.

optimize the structure of the charged water overlayer, Ab initio molecular dynamics simulations were conducted in canonical ensemble (NVT) with the Nose-Hoover thermostat [11][12][13] and a 1.0 fs time step at 300 K, as performed in our previous study 14 .
Reaction intermediates were included in the optimized geometry from AIMD simulations, and again to perform DFT calculations.Charge-density difference 15,16 was applied to investigate the charge transfer between doped Cd atom and Cu atom.where   and   are the potentials at the electrode surface and OHP, respectively; where   and   * are the charge and the bulk concentration of the  ℎ ionic species, respectively;  is the temperature of the electrolyte;  and  is the Faradaic constant and the ideal gas constant, respectively.The unevenly distributed charged ionic species could generate a non-uniform potential in the diffuse layer, which is denoted as

Supplementary
To take into account the steric effect, the effective solvent diameter of the charged species is considered in PB equation, which results in that the distribution of the ionic species in the diffuse layer follows equation (S3) when symmetric electrolyte (such as Furthermore, to account for the dependence of the dielectric permittivity on E-field, In equation (S4),  = |−∇()| is the local E-field strength,  is the electrolyte refractive index, and  is a parameter given by: 22 where  is the dipole moment of the water molecule.
The detailed simulation processes have been provided in our previous report.The one-time electrolyzer cost can be converted to a cost to generate one tonne of acetic acid.We assume the lifetime of the electrolyzer to be 20 years with no salvage value at the end of the plant's lifetime, as well as a plant capacity factor of 0.9 and a discount rate of 7%.The capital recovery factor (CRF) can be calculated.
The electrolyzer cost per tonne of acetic acid can be calculated using the CRF.Catalyst and membrane replacement cost.We assume the one-time catalyst and membrane replacement cost to be 5% of the total electrolyzer cost with a lifetime of 5 years and a discount rate of 7%.The total cost catalyst and membrane replacement cost can be calculated.
Similarly, the one-time catalyst and membrane replacement cost can be converted to a cost to generate one tonne of acetic acid.Electrolyte recovery and liquid production separation.The acetate is assumed to accumulate in the electrolyzer until a final concentration of 45.6 wt% (7.6 M) is achieved. 28The cost to protonate acetate to produce acetic acid and to recover the electrolyte is calculated using a similar method as reported. 28To briefly describe the process, alkali acetate is first protonated with an HCl solution to produce acetic acid and alkali chloride salt.It is assumed that all alkali cations are bound to acetate.The alkali chloride salt is then subsequently converted back to alkali hydroxide and HCl via electrolysis.A typical electricity demand of NaCl electrolysis (2000 kWh/tonne NaOH) is assumed for this process. 29An electricity price of 2 cent/kWh is used for the calculation.The electricity cost is assumed to be 50% of the total cost as reported for a typical chloralkali process.The separation cost is then studied using an ASPEN Plus model as described in Supplementary Note 3. The model estimates a capital cost of $ 11 845 500 and an operating cost of $ 56 433 per day for a production capacity of 650 tonne/day.
The capital cost can be converted to a cost to generate one tonne of acetic acid.We assume the lifetime of the electrolyzer to be 20 years with no salvage value at the end of the plant's lifetime, as well as a plant capacity factor of 0.9 and a discount rate of 7%.The operating cost can also be converted to a cost to generate one tonne of acetic acid.Input chemicals costs.The costs of input CO and water are calculated based on a CO price of $180 per tonne and a water price of $5 per tonne. 30We estimate that the energy to pressurize CO to 8 bar is ~1% of the electrolyzer electricity input and hence the cost of pressurization is not included.Installation costs.We assume a Lang factor of 1 for the equipment installation cost.The total capital costs are the sum of the capital cost of the electrolyzer, membrane and catalyst, and the distillation unit.Balance of plant (BoP).We assume the balance of plant is 50% of the total capital costs.The total capital costs are the sum of the capital cost of the electrolyzer, membrane and catalyst, and the distillation unit.

Supplementary Fig. 20 |Supplementary Fig. 21 |. 22 | 23 | 24 | 25 | 27 |
Products distribution in COR on Cd-Cu electrode in various 3 M alkali metal cations electrolyte under different current densities.(a) 3 M NaOH, (b) 3 M KOH and (c) 3 M CsOH.Products distribution in COR on Cd-Cu electrode in 2 M KOH + 1 M CsOH electrolyte under different current densities.Liquid products FE and acetate FE ratio in liquid products on the Cd-Cu and Cu electrodes in 2 M KOH + 1 M CsOH electrolyte under different current densities.Plot of electric filed near the electrode surface with different alkali metal cations.Behavior of interfacial water on Cd-Cu catalysts.(a) Operando ATR-SEIRAS spectra (grey curves) of the interfacial water on Cd-Cu in different electrolytes with varying ratios of K + and Cs + .These were fitted with three Gaussians (blue: ice like water; light blue: liquid like water; and yellow: free water, respectively).(b) The relationship between the area ratios of the three peaks at -1.7 V. Morphology characterization of the post-reaction Cd-Cu catalyst.(a) TEM image and (b) HR-TEM image of the used Cd-Cu catalyst after a 20 h stability test.We collected the post-reaction Cd-Cu sample after stability test and performed structural characterizations.TEM images show the well-preserved nanoplates morphology and no Cd nanoparticles are observed.Supplementary Fig. 26 | The AC HAADF-STEM image of the used Cd-Cu catalyst after a 20 h stability test.The intensity profiles along the white solid lines.The AC HAADF-STEM image suggests that no evidence of Cd agglomeration.The Cd single atoms are identified by the heightened intensity profiles of the areas marked in the AC HAADF-STEM image.The findings suggest that the structure and atomic dispersion of Cd are well-maintained, demonstrating a high stability of Cd-Cu.Energy efficiency and cell potential of Cd-Cu electrode under high pressure of 8 bar in 2 M KOH + 1 M CsOH electrolyte.

Supplementary Fig. 28 |
cation concentration and electric field were performed in the COMSOL Multiphysics package based on the finite-element-method solver (https://www.comsol.com/).The electric double layer at the electrode-electrolyte interface was modelled using the Gouy-Chapman-Stern (GCS) model17 , which consists of the stern layer and the diffuse layer.The Stern layer consists of a monolayer of surface-adsorbed hydrated cations on the electrode surface, with a thickness of   .The diffuse layer contains free anions and cations distributed according to the Poisson and Boltzmann law, which forms the concentration gradient away from the electrode surface.Simulation domain.Modified Poisson-Boltzmann (MPB) was applied to calculate the distribution of the cation and E-field at the electrode-electrolyte interface.The charge density at OHP,   , is calculated according to: 18   =  0   (  −   )   (S1) Acetate and ethylene FE ratio in COR products on the Cd-Cu and Cu electrodes under different current densities.
Supplementary Fig. 14 | The comparison of acetate FE on Cd-Cu catalysts with the different Cd loading.Supplementary Fig. 15 | Liquid products total FE on the Cd-Cu and Cu electrodes under different current densities.products Supplementary Fig. 17 | Operando ATR-SEIRAS spectra of (a) the Pd-Cu electrode, and (b) the Au-Cu electrode.Spectra presented correspond to 64 coadded scans collected with an 8 cm -1 resolution.a.u., arbitrary units.

Table 1 .
Performance comparison of CO toward acetate among stateof-the-art electrocatalysts.

Table 2 .
EXAFS fitting parameters at the Cd K-edge for Cd foil and Cd-Cu sample.CN: coordination numbers; b R: bond distance; c σ 2 : Debye-Waller factors; d ΔE0: the inner potential correction.R factor: goodness of fit.Ѕ0 2 was set as 0.78 for Cd data, which was obtained from the experimental EXAFS fit of Cd foil reference by fixing CN as the known crystallographic value and was fixed to all the samples. a

Table 3 .
Contributions to Gibbs free energies of gas species and adsorbates, including zero-point energy (ZPE), enthalpic temperature correction (∫   d ), entropy (S).H2 is referring to the energy used in computational hydrogen electrode model as described in Methods.
denotes the Stern layer thickness depending on the hydrated cation.The size of different hydrated cations is provided in Supplementary TableS5.Since there is no charge presented in the Stern layer, the potential is assumed to drop linearly from   to   in the Stern layer.

Table 4 .
Diffusion-coefficients in m 2 s -1 . 23Supplementary 30her operational costs.We assume other operational costs (such as labor and maintenance) to be 10% of the electrolyzer electricity cost.The plant-gate levelized cost to produce 1 tonne of acetic acid can be calculated by summing up all costs discussed earlier.The market price for acetic acid is assumed to be $600/tonne.30 Plant-gate levelized cost.